I believe this issue was fixed as the player, but when an NPC goes into the grass, he/she will rapidly be either in front of, or behind the grass, this is the best way I can prove it: two pause screens with an NPC nearby:
In this one, he is behind the grass:
In this one, he is in front of the grass:
Just note that he is actually doing this very quickly.
In this one, he is behind the grass:
In this one, he is in front of the grass:
Just note that he is actually doing this very quickly.